At present, foamed-crosslinking type urethanes are widely used a cushioning material that is used for furniture, beddings such as beds, and seats for vehicles such as those for trains, automobiles and two-wheeled vehicles.
Although foamed-crosslinking type urethanes have excellent durability as a cushioning material, they have inferior moisture and water permeability and air permeability, and have thermal storage property to exhibit possible humid feeling. Since the foamed-crosslinking type urethanes do not have thermoplasticity, they have difficulty in recycling, and therefore they give significant damage to incinerators in case of incineration, and need high costs in elimination of poisonous gas. For this reason, the foamed-crosslinking type urethanes are often disposed of by landfill, but limitation of landfill spots based on difficulty of stabilization of ground causes problems of the necessity for higher costs. Furthermore, although the foamed-crosslinking type urethanes have excellent workability, they may cause various problems such as pollution problems with chemicals that have been used in the manufacturing process, residual chemicals after foaming and associated offensive odors.
Patent Documents 1 and 2 disclose network structures. They are capable of solving various problems associated with the foamed-crosslinking type urethanes and have excellent cushioning performance. As for repeated compression durability properties, however, only the 50%-constant displacement repeated compression residual strain is excellent. The 50%-compression hardness retention after 50% repeated compression is about 83%, thus a hardness is low after repeated use.
Conventionally, if the repeated compression residual strain is low, durability performance is considered to be sufficient. However, in recent years, requirements for repeated compression durability have become higher. Thus, a 40%-compression hardness retention after 750 N-constant load repeated compression equivalent to a human body weight of about 76 kgs is emphasized instead of an evaluation of 50%-constant displacement repeated compression durability, and requirements for improving the constant load repeated compression durability have become higher. In a conventional network structure, a 40%-compression hardness retention after 750 N-constant load repeated compression is only about 50%, and the improvement thereof is desired. However, it is difficult to obtain, from a conventionally known network structure, a network structure having a high hardness retention after constant load repeated compression.
Patent Document 3 discloses a network structure that includes fibers which are different in fineness and a method for manufacturing the same. In this structure and the method, for the surface layer and the base layer, a ratio in sectional secondary moment of a circular cross-sectional area is used to define a difference in fineness, and a soft layer having a small fiber diameter is provided in the surface and an inner layer in which a fiber diameter is large so as to have durability is provided as the base layer, thereby improving cushioning performance and durability. This manufacturing method is excellent conventionally for a 50%-constant displacement repeated compression. However, the manufacturing method, the structure, or the like are not necessarily excellent for a 750 N constant load repeated compression durability that is tested more strictly and is to be achieved by the present invention, and it is difficult to achieve the scope of the present invention thereby.